Cracks reveal earthquake history in northern Chile - Miami geologist part of study

Jason Rech, associate professor of geology (left), prepares to enter a surface crack in the Atacama Desert. Surface cracks in this very dry area opened during great earthquakes and represent thousands of earthquakes over nearly one million years (photo courtesy Jason Rech)

Earth's crust is generally considered to rebound following an earthquake so that deformation away from the fault is not permanent. However, a team of researchers, including Miami geologist Jason Rech, has identified numerous large cracks in northern Chile’s Atacama Desert that widen during successive earthquakes. These results indicate that the crust has been permanently deformed by thousands of earthquakes.

Their study, “Permanent deformation caused by subduction earthquakes in northern Chile,” was published in Nature Geoscience April 28 (online ahead of print).

Earthquakes are accompanied by seismic rebound: blocks of crust on either side of the fault spring back to their initial, undeformed configuration, explain the researchers. “This rebound is well documented by space geodetic data, such as the Global Positioning System. Thus, all earthquake-induced deformation of the crust is considered non-permanent” and is modeled as an elastic process.

Their research, however, shows that earthquakes larger than magnitude 7 in northern Chile - one of the world’s most earthquake-prone places - caused the crust to deform permanently.

The Atacama Desert may be the only place in the world where permanent deformation caused by earthquakes - such as these cracks - can be preserved and identified (photo courtesy Jason Rech)

In most locations on Earth earthquakes rupture and deform the landscape surface, said Rech, associate professor of geology and environmental earth science. “Features associated with the earthquakes are quickly erased from the landscape by erosion and deposition, at least in areas where it rains more than once a decade.”

Any permanent deformation would normally be exceedingly difficult to detect in the geological record.

The Atacama Desert -the driest place on Earth - has been hyper-arid for millions of years, Rech explained. Because it is so dry and stable it is the best Earth analog for Mars. Surface cracks in this very dry area opened during great earthquakes and represent thousands of earthquakes over nearly one million years.

The Atacama Desert may be the only place in the world where such permanent deformation can be preserved and identified, say the study authors.

“The brittle soils of the Atacama Desert allow us to reconstruct the nature of typical earthquakes – key for predicting seismic hazards – and to document deformation,” Rech said.

“Typical earthquake modeling follows the elastic rebound theory,” said Rick Allmendinger, professor of earth and atmospheric sciences at Cornell University and lead author of the study.

However, the researchers show that up to 10 percent of the horizontal deformation generated during the earthquakes, recorded by Global Positioning System data and previously assumed to be recoverable, is permanent.

They discovered this by looking at geodetic GPS data that records, down to a sub-centimeter scale, changes in the earth’s surface in real time – a technology that has revolutionized all of earth sciences, Allmendinger said.

Jason Rech and the field team documented surface deformation due to thousands of earthquakes in Iquique Gap. They surveyed about 11 kilometers (almost seven miles) of scan lines across cracks in the area, measuring their width and orientation. They used a method called cosmogenic nuclide dating determine the ages of the surface of the earth on both sides of the cracks (photo courtesy Jason Rech)

The field team documented surface deformation due to thousands of earthquakes in Iquique Gap, an area along the subduction zone where there has not been a large earthquake since the late 1800s and therefore is expected to be the location of a great earthquake soon. They surveyed about 11 kilometers (almost seven miles) of scan lines across cracks in the area, measuring their width and orientation. They used a method called cosmogenic nuclide dating to determine the ages of the surface of the earth on both sides of the cracks. They then calculated the strain rate of the area, which allowed them to compare it to the modern GPS record.

“By using these cracks in this very dry area, we actually now have a statistically significant sample of thousands of earthquakes, because these cracks are the records of thousands of earthquakes,” Allmendinger said.

The research team’s data set provides a record of permanent strain in the shallow crust of the South American Plate. They state that “although deformation of the deep crust may be predominantly elastic, we conclude that modeling of the earthquake cycle should also include a significant plastic component.”

The research team was led by scientists Allmendinger and included Amanda Baker, former doctoral student at Cornell University and first author of the paper, Rech and Lewis Owen, professor of geology at the University of Cincinnati.

The collaborative study led by U.S. Geological Survey that cast doubt on a contested possible extraterrestrial impact 12,900 years ago. Instead, the team suggested the environmental markers tied to climate change and extinctions from that time period are the result of natural processes.

Closer to Oxford: Summer Workshop, canoeing down the Whitewater River

Rech will be leading a new summer workshop, “Geology of Streams in Ohio and Indiana” in partnership with Miami’s Outdoor Pursuit Center May 16-23.

Rech and students will explore the impact of climate change and historic land use on streams while canoeing down the Whitewater River.